We present a nonperturbative QCD calculation of diffractive vector meson production in virtual photon nucleon scattering at high energy. We use the nonperturbative model of the stochastic QCD vacuum which yields linear confinement and makes specific predictions for the dependence of high-energy scattering cross sections on the hadron size. Using light cone wave functions of the photon and vector mesons, we calculate electroproduction cross sections for ρ, ω, φ and J/ψ. We emphasize the behavior of specific observables such as the ratio of longitudinal to transverse production cross section and the t-dependence of the differential cross section.

We study the p-wave polarization operator of the ρ-meson due to ρN interactions via the N * (1720) and ∆(1905) resonances and compute the corresponding production rate for e + e − -pairs at finite temperature and baryon density. At high baryon density we find a significant shift of the spectrum to lower invariant masses.

A model combining perturbative and non-perturbative QCD is developed to compute high-energy reactions of hadrons and photons and to investigate saturation effects that manifest the S-matrix unitarity. Following a functional integral approach, the S-matrix factorizes into light-cone wave functions and the universal amplitude for the scattering of two color-dipoles which are represented by Wegner-Wilson loops. In the framework of the non-perturbative stochastic vacuum model of QCD supplemented by perturbative gluon exchange, the loop-loop correlation is calculated and related to lattice QCD investigations. With a universal energy dependence motivated by the twopomeron (soft + hard) picture that respects the unitarity condition in impact parameter space, a unified description of pp, πp, Kp, γ * p, and γγ reactions is achieved in good agreement with experimental data for cross sections, slope parameters, and structure functions. Impact parameter profiles for pp and γ * L p reactions and the gluon distribution of the proton xG(x, Q 2 , | b ⊥ |) are calculated and found to saturate in accordance with S-matrix unitarity. The c.m. energies and Bjorken x at which saturation sets in are determined.

Predictions for semi-inclusive deep inelastic lepton-nucleus scattering are presented. Both the effects of gluon radiation by the struck quark and the absorption of the produced hadron are considered. The gluon radiation covers a larger window in virtuality Q 2 because of the increased deconfinement of quarks inside nuclei. The absorption of hadrons formed inside the nucleus is described with a flavor dependent cross section. Calculations for rescaled fragmentation functions and nuclear absorption are compared with the EMC and HERMES data for N, Cu and Kr targets with respect to the deuteron target. Predictions for Ne and Xe targets in the HERMES kinematic regime are given.